X-ray imaging is a well known and extremely valuable tool for the early detection and diagnosis of various disease states in the human body. The use of contrast agents for image enhancement in medical x-ray imaging procedures is widespread. An excellent background on iodinated and other contrast agents for medical imaging is provided by D. P. Swanson et al, Pharmaceuticals in Medical Imaging, 1990, MacMillan Publishing Company.
U.S. Pat. No. 3,097,228 describes derivatives of 2,4,6-triiodobenzoyloxyalkanoic acids having the structure ##STR2## wherein R.sup.1 is H or lower alkyl; R.sup.2 is H or lower alkanoyl; R.sup.3 is H or lower alkanoylamino and R.sup.4 is lower alkyl. The agents are useful as x-ray contrast agents for visualizing the gall bladder (cholecystography) when administered orally, in the free acid form or in the form of a non-toxic salt, or intravenously, in the form of water soluble, non-toxic salt. Example 15 therein describes ethyl 2-(3,5-diacetamido-2,4,6-triiodobenzoyloxy)hexanoate, i.e., ##STR3##
Bacon et al, commonly assigned U.S. patent application Ser. No. 07/990,987 filed Dec. 16, 1992 describes iodinated aroyloxy esters which are useful as contrast agents in x-ray imaging compositions and methods. However, all of the compounds described by Bacon et al feature an ester group linked through a C.sub.2 or higher alkylene group to another ester group on an iodinated aromatic ring.
EP-A 498,482 describes nanoparticulate x-ray contrast compositions which have proven to be extremely useful in medical imaging. The compositions comprise particles of an organic x-ray contrast agent and a surface modifier adsorbed on the surface thereof and have an effective average particle size of less than 400 nm. The agents can be delivered to a specific tissue or fluid site, e.g., the blood pool, liver, spleen, kidney or lymph nodes. Example 8 therein describes a formulation comprising ethyl 2-(3,5-bis(acetylamino)-2,4,6-triiodobenzoyloxy)butyrate, i.e., ##STR4## wherein (Z--COO is the residue of diatrizoic acid.
However, it has been discovered that ethyl 2-(3,5-bis(acetylamino)-2,4,6-triiodobenzoyloxy)butyrate exhibits multiple crystal forms, i.e., polymorphs, e.g., when recrystallized from various solvents. The reasons for this behavior are not completely understood but, in any event, multiple crystal forms are disadvantageous for a variety of reasons. For example, the presence of multiple crystal forms renders scale up problematic due to the lack of reproducibility of the results obtained, including, e.g., in chemical manufacturing and in the milling process. Additionally, it has been found that nanoparticulate formulations of ethyl 2-(3,5-bis(acetylamino)-2,4,6-triiodobenzoyloxy)butyrate do not exhibit good stability during autoclaving, i.e., conventional heat sterilization.
Consequently, it would be highly desirable to provide a poorly soluble x-ray contrast agent having the advantages of ethyl 2-(3,5-bis(acetylamino)-2,4,6-triiodobenzoyloxy)butyrate but which exhibits a consistent and reproducible crystal morphology, is amenable to reproducible scale up and can be successfully heat sterilized by autoclaving.